MEXICO CITY – With Mexico snarled in political uncertainty, the leading leftist candidate headed to court Sunday evening to begin a legal fight against what appears to be a razor-thin victory by his conservative rival in the presidential election a week ago.

Andres Manuel Lopez Obrador said his lawyers would give the Federal Electoral Court evidence of fraud, including computerized manipulation of the results, a day after he held a mammoth rally in Mexico City's historic center and called on his supporters to help keep his hope alive.

The legal appeal would not seek to annul the July 2 election, but to force authorities to conduct a manual recount of all 41 million ballots. Over 100,000 supporters rallying in the capital's main plaza Saturday chanted "Vote by vote!"

Election officials said Thursday that Felipe Calderon beat Lopez Obrador by less than 244,000 votes in the July 2 election — or a margin of just 0.6 percent.

But Lopez Obrador contends some of his votes weren't counted or were voided without reason. He has millions of devoted followers who believe only he can help Mexico's poor and downtrodden, and he has long used street protests to pressure the government and courts.

Fox has denied interfering in the elections, and election monitors from the European Union said they found no irregularities in the vote count.

But fraud allegations strike a sensitive nerve with many Mexicans. They question whether Mexico has overcome decades of institutional corruption and fraud that long favored the ruling party — the Institutional Revolutionary Party for 71 years until it lost the presidency to Fox in 2000.

Lopez Obrador has sought to stoke those fears. On Saturday, he accused the respected Federal Electoral Institute, held up as an example to emerging democracies around the world, of being a "pawn of the party of the right."

Turning to his charges that Fox unfairly aided Calderon, he said the popular president "dedicated himself to attacking us and ended up being a complete traitor of democracy."

Representatives for Fox and Calderon were not immediately available to comment. Fox has stayed out of the public eye for two weeks in an effort to avoid accusations that he was meddling in the debate.

Lopez Obrador, a former Mexico City mayor, further said a software program was used to skew initial vote-count reports — a charge that revives memories of the alleged computer crash that flipped Mexico's 1988 election in favor of the then-ruling PRI.

Legal challenges were built into Mexico's elections process in recent years to help ensure clean elections, so Calderon can't be declared president-elect until the electoral court weighs allegations of fraud or unfair campaign practices. The court has until Sept. 6 to declare a winner.

The law allows a manual recount only for polling places where credible evidence of irregularities exist. Lopez Obrador's supporters say that applies to at least 50,000 of the approximately 130,000 polling stations.

"We are going to ask that they clean up the elections. We are going to ask that they count all the votes, vote-by-vote, poll-by-poll," Lopez Obrador said.

Despite the EU monitoring team's characterization of the vote count as fair, the leftist claimed election fraud was worse than under the PRI, which often using fraud to hold power.

Calderon says the vote was clean and has taken congratulatory phone calls from President Bush and the leaders of Canada, Spain and Colombia, among others — despite Lopez Obrador's plea for foreign governments to hold off on recognizing the result.

Lopez Obrador has repeatedly evaded questions about whether he would accept a court decision that went against him.

He provoked groans of disappointment from the crowd at the Zocolo in Mexico City when he told them not to block highways.

"This has been and goes on being a peaceful movement," he said. "We are not going to fall for any provocations."

Also Sunday, the Federal Electoral Institute will certify the results of the July 2 election and wrap up vote tallies that will determine the number of seats each party has in Congress.

According to calculations by Mexican news media based on a preliminary vote count, Calderon's party would get 210 seats in the lower house, Lopez Obrador's leftist alliance 163 seats and an alliance led by PRI 113 seats. Fourteen seats will go to minor parties